1. Field of the Invention
The present invention relates to electrical steel sheets having superior magnetic properties, anti-noise properties, and workability, which are suitably used as compact iron core materials primarily for use in compact transformers, motors, electric-generators, and the like. The invention also relates to methods for manufacturing such electrical steel sheets.
2. Description of the Related Art
Compact iron core materials in electric apparatuses are mainly required to have superior magnetic properties. In addition, superior anti-noise properties or superior workabilities are desired.
Magnetic properties will first be described. Magnetic properties are greatly influenced by the orientations of crystalline grains constituting steel sheets. Among the directions mentioned above, it has been well known that, in order to obtain superior magnetic properties, the  less than 001 greater than  axes, i.e., the axes of easy magnetization of crystalline grains, should be parallel with the surface of the steel sheet.
The following types of steel sheet are conventionally used for iron cores in compact electric apparatus: (1) a general-purpose cold-rolled steel sheet or a decarburized steel thereof, (2) a non-oriented silicon steel sheet in which the iron loss is decreased by adding silicon (Si) and by decreasing impurities; (3) a singly oriented silicon steel sheet in which crystalline grains are preferentially grown having the Goss orientations, i.e., the {110} less than 001 greater than  orientation, by using secondary recrystallization; and (4) a doubly oriented silicon steel sheet in which crystalline grains are preferentially grown having the cube orientations, i.e., the {100} less than 001 greater than  orientation.
Among the steel sheets described above, the general-purpose cold-rolled steel sheet, the decarburized steel sheet thereof, and the non-oriented silicon steel sheet have a smaller number of crystalline grains in the surface thereof having the  less than 001 greater than  axes in parallel with each other since the evolution of the texture is insufficient. Accordingly, compared to the singly oriented silicon steel sheet, superior magnetic properties cannot be obtained.
The singly oriented silicon steel sheet is most generally used for iron core materials for transformers. In the singly oriented silicon steel sheet composed of crystalline grains integrated in the Goss orientations, the  less than 001 greater than  axes, which are easily magnetized, are highly integrated in a rolling direction. Consequently, in particular, when magnetization is performed in the rolling direction, superior magnetic properties can be obtained. However, the  less than 111 greater than  axes, which are most difficult to magnetize, are present in the surface of the steel sheet. As a result, when magnetization is performed in the direction of the axes described above, the magnetic properties are extremely inferior. That is, singly oriented silicon steel sheets are advantageously used for applications, such as for transformers, which require superior magnetic properties only in one direction. On the other hand, singly oriented silicon steel sheets are not advantageously used for applications, such as for iron core materials for motors and electric generators or the like, which require superior magnetic properties in multiple directions on the surface of the steel sheet.
Methods for manufacturing doubly oriented silicon steel sheets have been researched for many years, in which the cube-oriented texture is grown by secondary recrystallization. For example, a method is disclosed in Japanese Examined Patent Application Publication No. 35-2657, in which the cube-oriented grains are recrystallized by so-called xe2x80x9ccross rollingxe2x80x9d while using an inhibitor. In the method described above, secondary recrystallization is performed by cross rolling in which cold rolling is performed in one direction followed by cold rolling in the direction perpendicular thereto, annealing for a short period, and annealing at a higher temperature of 900 to 1,300xc2x0 C. In addition, a method is disclosed in Japanese Unexamined Patent Application Publication No. 4-362132, in which the cube-oriented grains are recrystallized using aluminum nitride (AlN) after cold rolling is performed at a reduction rate of 50 to 90% in the direction perpendicular to hot rolling direction. In the method described above, after cold rolling, annealing is conducted so as to perform primary recrystallization, and final finish annealing is then conducted so as to perform secondary recrystallization and purification.
In the methods performed using recrystallization, steel sheets having cube-oriented texture are obtained in which the  less than 100 greater than  axes in the surface thereof are highly integrated in the rolling direction. Accordingly, magnetic properties in the rolling direction and the direction perpendicular thereto are superior. However, as the direction 45xc2x0 with respect to the rolling direction is the  less than 110 greater than  axes orientation, which is difficult to magnetize, the magnetic properties in this direction are inferior.
In the steel sheets having the {100} orientations in the rolling surfaces thereof, a number of the easily magnetized axes  less than 100 greater than  are present in the rolling surface, and the difficult magnetization axes  less than 111 greater than  are not present. Accordingly, compared to the steel sheets conventionally used, the steel sheets having the {100} orientations in the rolling surfaces can be advantageously used for applications which require superior magnetic properties in every direction in the surfaces thereof. In particular, in the steel sheet composed of crystals having the {100} less than uvw greater than  orientations in which the rolling surface is in parallel with the {100}orientation, and the  less than 001 greater than  axes are randomly aligned in the rolling surface, ansiotropic magnetic properties are not present at all in the rolling surface direction. Therefore, the steel sheets described above are ideal materials for use in motors.
Based on the understanding described above, methods for growing the {100} texture have been attempted. In the present invention, xe2x80x9cto grow the {100} texturexe2x80x9d means xe2x80x9cto increase the number of crystals having the {100} orientations forming a rolling surface.xe2x80x9d
For example, a method is disclosed in Japanese Examined Patent Application Publication No. 51-942, in which cold rolling is performed at a reduction rate of 85% or more, and more preferably, 90% or more, and after that, prolonged annealing is performed at 700 to 1,2000xc2x0 C. for 1 minute to 1 hour. However, in the method described above, even though the {100} texture is grown immediately after rolling is complete, the {111} texture is also grown after prolonged annealing for recrystallization is performed. As a result, the product thus formed cannot is have superior magnetic properties.
In addition, a method is disclosed in Japanese Examined Patent Application Publication No. 57-14411, in which, after cold rolling is complete, a cooling rate is controlled in the phase transition region from a xcex3 phase to an xcex1 phase during recrystallization. However, in the method described above, since a xcex3 transformation must occur during recrystallization, the content of Si, which stabilizes the xcex1 phase, cannot be increased. For example, when carbon (C) and manganese (Mn) are not contained, the xcex3 transformation will not occur when the content of Si is approximately 2 wt % or more, whereby the method cannot be used. That is, the method described above is a disadvantageous method since the content of Si cannot be increased, which also advantageously works to decrease an iron loss.
Furthermore, a method is disclosed in Japanese Unexamined Patent Application Publication No 5-5126, in which a steel containing 0.006 to 0.020 wt % C is cold rolled, is recrystallized by heating to 900 to 1,000xc2x0 C., and is subsequently processed by recrystallization annealing. The steel sheet thus obtained according to Example 1 in the same publication described above has a magnetic flux density B50 of approximately 1.66 to 1.68 T, which is an average of the values obtained in the rolling direction and the direction perpendicular thereto. That is, the  less than 001 greater than  axes in the surface of the steel sheet are not so highly integrated.
As described above, conventional methods for manufacturing non-oriented silicon steel sheets do not sufficiently grow the {100} texture, Consequently, the magnetic properties cannot be sufficiently improved.
FIG. 1 shows an EI core, which is a typical shape of a compact transformer formed of laminated steel sheets.
As an iron core material used for the EI core, both non-oriented silicon steel sheets and singly oriented silicon steel sheets are presently used.
When a non-oriented silicon steel sheet is used, compared to the case in which a singly oriented silicon steel sheet is used, the magnetic properties of the core are inferior thereto. The reason for this is that the magnetic properties of a non-oriented silicon steel sheet are inferior to those of a singly oriented silicon steel sheet. However, compared to a singly oriented silicon steel sheet, a non-oriented silicon steel sheet is used from an economic point of view, since it can be produced by a simpler manufacturing process and is lower in cost.
In contrast, a singly oriented silicon steel sheet has superior magnetic properties in the rolling direction but has extremely inferior magnetic properties in the direction perpendicular thereto. When a singly oriented silicon steel sheet is used as an iron core material for the EI core, the flow of magnetic flux is both in the rolling direction and the direction perpendicular thereto. Compared to a non-oriented silicon steel sheet, the magnetic properties of the core composed of a singly oriented silicon steel sheet is superior; however, the singly oriented silicon steel sheet is not advantageously used.
It is believed that a doubly oriented silicon steel sheet, which has superior magnetic properties in both the rolling direction and the direction perpendicular thereto, is most advantageous. However, in the conventional methods, cross rolling is required for manufacturing a doubly oriented silicon steel sheet, such that production yield is extremely low. Such products have not been made on an industrial mass production scale. In addition, in iron cores used for compact transformers, such as an EI core, a portion at which the flow of magnetic flux changes orthogonally will have significant influence. In other words, a doubly oriented silicon steel sheet cannot be an ideal material, since the magnetic properties in the direction oriented 45xc2x0 away from the rolling direction are inferior.
As described above, the conventional methods do not produce an ideal iron core material, such as an EI core in compact transformers.
Next, anti-noise properties will be described. Recently, especially considering environmental issues, concomitant with even more strict regulations for controlling noise, noise generated by transformers and the like is increasingly a serious problem. Accordingly, reduction in noise generated by transformers is an essential requirement therefor.
Consequently, manufacturers of transformers are very interested in magnetostriction properties, which are considered to be a major reason for generating noise, and have requested material manufacturers to decrease the noise generation. As a result, in order to respond to the requirements described above, the material manufacturers have made intensive efforts to reduce magnetostriction of electrical steel sheets.
It is believed that magnetostriction is caused by, when a steel sheet is magnetized, movement of 90xc2x0 magnetic domain walls and a rotating magnetization. Consequently, magnetostriction is effectively reduced when 90xc2x0 magnetic domains are decreased.
In singly oriented silicon steel sheets, by enhancing orientations of crystalline grains using an inhibitor or the like, reduction in magnetostriction, in addition to improvement in magnetic properties, is achieved. The reduction in magnetostriction is achieved by increasing 180xc2x0 magnetic domains and by decreasing 90xc2x0 magnetic domains.
In order to further reduce magnetostriction, a method is conventionally employed in which a film or an insulating coating, which can impart tensile force, is used. The method described above is a method exploiting a phenomenon in which, when tensile force is provided to a steel sheet, the widths of 180xc2x0 magnetic domains are decreased, and 90xc2x0 magnetic domains are decreased. That is, this method is a method in which an insulating coating is formed on a steel sheet by baking at a higher temperature, and tensile force is imparted to the steel sheet by using a difference in coefficients of thermal expansion between the steel sheet and the insulating coating, whereby the magnetostriction is reduced.
For example, a method for forming a tensile coating composed of colloidal silica, aluminum phosphate, and chromic anhydride is disclosed in Japanese Examined Patent Application Publication No. 53-28375. In addition, a method is disclosed in Japanese Examined Patent Application Publication No. 5-77749, in which at least one thin film of TiC, TiN, and Ti(C,N) is adhered to a steel sheet so as to impart tensile force thereto. However, since most of the tensile films and tensile coatings are composed of glass materials or ceramic materials, there are problems in that they are brittle and are easily separated during stamping. As a result, the methods described above can be applied only to singly oriented silicon steel sheets in which almost no stamping properties are required, and in practice, the methods described above cannot be applied to electrical steel sheets in which stamping properties are essential.
A phenomenon is known in which, when the content of Si in a Fexe2x80x94Si alloy is close to 6 wt %, the magnetostriction constants xcex100 and xcex111 are nearly zero, and magnetostriction will not occur. By exploiting the phenomenon described above, in order to improve magnetostriction properties, a method of increasing the content of Si is attempted.
For example, a method is disclosed in Japanese Unexamined Patent Application Publication No. 62-227078, in which Si is impregnated in a steel sheet containing less than 4 wt % Si, and Si is diffused in the sheet thickness direction, thereby yielding a high silicon steel sheet. However, when the content of Si in a steel sheet is increased, the fabrication properties of the steel sheet are extremely degraded. As a result, the method described above is difficult to apply to steel sheets which are formed into iron cores for motors or the like by stamping. Furthermore, in the method described above, impregnation of Si cannot be performed uniformly, and hence, non-uniformity can be observed in the sheet thickness direction, which cannot be ignored. As a result, problems may arise in that magnetic properties and magnetostriction are difficult to control.
In addition, in Japanese Unexamined Patent Application Publication Nos. 9-275021 and 9-275022, methods are disclosed in which low noise iron cores are obtained by setting the absolute value of direct current magnetostriction of non-oriented silicon steel sheets to be 1.5xc3x9710xe2x88x926 or less. In the method described above, in order to set the absolute value of direct current magnetostriction of non-oriented silicon steel sheets to be 1.5xc3x9710xe2x88x926 or less, it is clearly described that the content of Si is controlled to be 4.0 to 7.0 wt %. However, when Si is contained at a high concentration in a steel sheet as described above, the fabrication properties thereof are extremely degraded. As a result, the method is difficult to apply to steel sheets which are formed into iron cores for motors or the like by stamping.
Finally, workability will be described. In particular, in a steel sheet in which a large number of the cube-oriented grains represented by the Miller index of the {100} less than 001 greater than  is present, it is considered that the workability thereof is extremely degraded. The steel sheet described above is represented by a doubly oriented silicon steel sheet, and the magnetic properties thereof are degraded by fabrication more seriously than those of a singly oriented and a non-oriented silicon steel sheet, that is, the workability is degraded.
The reason for this is that the conventional doubly oriented silicon steel sheet formed by exploiting secondary recrystallization has crystalline grains having diameters significantly larger than those of the non-oriented silicon steel sheets. As a result, edge portions of the conventional doubly oriented silicon steel sheet are likely to deform during cutting and stamping, and hence, larger distortions are likely to be generated. In addition, by finish annealing at a higher temperature, a rigid oxide film primarily composed of forsterite is formed. The rigid film increases the distortions at the edge portions of the steel sheet. As a result, the magnetic properties are degraded by the distortions described above.
In order to solve the problems described above, Japanese Unexamined Patent Application Publication No. 5-275222 proposes that a non-magnetic oxide on a surface is reduced by pickling, polishing, or the like. However, by reducing only a non-metal material on surfaces, insulation properties between steel sheets are degraded. In the method described above, the magnetic flux density is increased; however, the iron loss is also increased, and hence, materials according to the method are not preferably used as iron core materials. In addition, in pickling or polishing, since the oxide may be non-uniformly removed, or since distortion may be newly introduced, the iron loss is degraded.
On the other hand, in a singly oriented silicon steel sheet formed by exploiting secondary recrystallization, similarly to the above, tensile force is imparted to the steel sheet by a forsterite film and a silica-phosphate-based coating. As a result, the influence of distortion is alleviated.
However, when a tensile coating as described above is applied to a doubly oriented silicon steel sheet, magnetic properties in one of the rolling direction (L direction) and the direction perpendicular thereto (C direction) are improved, but magnetic properties in the other direction, which are not improved, are degraded. In polycrystalline doubly oriented silicon steel sheets manufactured in industrial production process, orientations of crystalline grains vary. Accordingly, magnetic properties in only one of the L direction and the C direction, in which the  less than 001 greater than  axes are highly integrated, are preferentially improved by a tensile coating, but in contrast, magnetic properties in the other direction are degraded.
The problems relating to the workability of the doubly oriented silicon steel sheets can be applied to a steel sheet in which a ratio of the cube oriented grains is high, according to the mechanism thereof.
As has thus been described, in view of magnetic properties, economic considerations, and the like, there has yet to be manufactured on any commercial scale an electrical steel sheet that is ideal for use as an iron core material in compact electric apparatuses.
The present invention solves the problems described above. An object of the present invention is to provide a totally new electrical steel sheet in compact iron cores, which has the most desirable magnetic properties and is advantageous in view of economic considerations, and to provide a manufacturing method therefor. In addition, another object of the present invention is to provide an electrical steel sheet having superior anti-noise properties and superior workability in which degradation of the magnetic properties is suppressed which is caused by distortion in fabrication, and to provide a manufacturing method therefor.
According to the present invention, an electrical steel sheet comprises from about 2.0 to about 8.0 wt % Si, from about 0.005 to about 3.0 wt % Mn, from about 0.0010 to about 0.020 wt % aluminum (Al), balance essentially iron, wherein the magnetic flux density B50(L) in the rolling direction and the magnetic flux density B50(C) in the direction perpendicular to the rolling direction are about 1.70 T or more, and the ratio B50(L)/B50(C) is from about 1.005 to about 1.100. In the electrical steel sheet according to present invention, secondary recrystallized grains inclined by 20xc2x0 or less with respect to the {100} less than 001 greater than  orientation are preferably present in the steel sheet at an areal ratio of 50% to 80%, and secondary recrystallized grains inclined by 20xc2x0 or less with respect to the {110} less than 001 greater than  orientation are preferably present in the steel sheet at an areal ratio of 6% to 20%. The electrical steel sheet according to the present invention may further comprise at least one member selected from the group consisting of nickel (Ni), tin (Sn), antimony (Sb), copper (Cu), molybdenum (Mo), and chromium (Cr). In order to improve the anti-noise properties, in the electrical steel sheet according to the present invention, the sum of the magnetostrictions in the rolling direction and in the direction perpendicular thereto is preferably set to be 8xc3x9710xe2x88x926 or less, and secondary recrystallized grains inclined by 15xc2x0 or less with respect to the {100} less than 001 greater than  orientation are preferably present in the steel sheet at an areal ratio of 30% to 70%. In order to avoid the degradation of the properties during fabrication, an amount of an oxide formed on the surface of the steel sheet is preferably controlled to be 1.0 g/m2 or less as an amount of oxygen on one surface of the steel sheet apart from an insulating coating, or tensile force of the oxide on the surface of the steel sheet and a coating formed on the steel sheet, which is imparted to the steel sheet, is preferably 5 MPa or less.
In addition, a method for manufacturing an electrical steel sheet according to the present invention, comprises steps of; hot rolling a steel slab containing from about 0.003 to about 0.08 wt % C, from about 2.0 to about 8.0 wt % Si, from about 0.005 to about 3.0 wt % Mn, and from about 0.0010 to about 0.020 wt % Al; annealing the hot-rolled steel sheet at a temperature of from about 950 to about 1,200xc2x0 C. when necessary; cold rolling at least once the hot-rolled steel sheet or the annealed steel sheet, in the case in which a cold rolling is performed two times or more, an intermediate annealing is performed therebetween; recrystallization annealing the cold-rolled steel sheet; coating a separator for annealing on the steel sheet processed by the recrystallization annealing step when necessary; final finish annealing the steel sheet processed by the recrystallization annealing to a temperature range of about 800xc2x0 C. or more; flattening annealing the steel sheet annealed by the final finish annealing step when necessary; and forming a insulating coating on the steel sheet. In addition, in the method for manufacturing the electrical steel sheet according to the present invention, the contents of sulfur (S) and selenium (Se) are preferably controlled to be 100 ppm by weight or less, respectively, the contents of nitrogen (N) and oxygen (O) are preferably controlled to be 50 ppm by weight, respectively, which are unavoidable impurities, the average heating rate is preferably set to be 30xc2x0 C./hour or less above 750xc2x0 C. in the final finish annealing step, and the steel slab preferably further comprises at least one member selected from the group consisting of Ni, Sn, Sb, Cu, Mo, and Cr. In order to improve the anti-noise properties, the recrystallization annealing step is preferably performed at a temperature of 800 to 1,000xc2x0 C. in an atmosphere in which a ratio of nitrogen is 5 vol % or more. In order to avoid degradation of the properties caused by fabrication, it is preferable that the average diameter of crystalline grains be set to be 200 xcexcm or more before a final cold rolling step is performed, the reduction rate in the final cold rolling step be set to be 60 to 90%, and the final finish annealing step be performed at 1,100xc2x0 C. or less in an atmosphere in which the dew point is 10xc2x0 C. or less and a volume percentage of oxygen is 0.1% or less. In addition, forming insulating coating step is preferably performed by coating an organic coating material at a thickness of 5 xcexcm or less, a semi-organic coating material, composed of an organic resin and an inorganic component, at a thickness of 5 xcexcm or less, or an inorganic glass coating material at a thickness of 2 xcexcm or less.